The present invention generally relates to noise reducing apparatuses, and more particularly to a noise reducing apparatus capable of preventing generation of noise modulation phenomenon, generally referred to as breathing, when reducing noise by compression and expansion of signal levels.
Conventionally, noise reducing apparatuses have been known which perform compression of the signal level, and expansion of the signal level complementary to the signal level compression, in order to reduce noise (hiss noise and the like) introduced upon reproduction of a magnetic recording medium in a magnetic recording and/or reproducting system. These known noise reducing apparatuses were used for reducing noise (surface noise and the like) introduced upon tracing of a record disc during reproduction in a recording and reproducing system for the record disc, and further used for improving degradation introduced in a ratio between receiving signal level and receiving noise level (signal-to-noise ratio, or simply S/N ratio) upon long-distance signal reception and the like in the field of signal transmission and reception of radio broadcasting.
The above type of a noise reducing apparatus generally comprises a level compression circuit having a variable gain control circuit supplied with an input signal and a control loop circuit which produces a control voltage for controlling an output signal of the variable gain control circuit to control the gain of the variable gain control circuit, and a level expansion ciruit having a variable gain control circuit supplied with a recorded and reproduced (or transmitted and received) signal and a control loop circuit for controlling the gain of this variable gain control circuit within the level expansion circuit. The noise within the level compressed signal applied to the level expansion circuit is subjected to level modulation by the level expansion circuit, since the control voltage level varies according to a level expansion characteristic in accordance with the level variation in the level compressed signal. Therefore, noise modulation phenomenon was introduced in the output of the level expansion circuit.
Hence, in the above conventional noise reducing apparatus, degradation is introduced in the auditory fidelity if the level modulated noise obtained from the level expansion circuit is not masked by the signal level.
The above noise modulation phenomenon is closely related to the recovery time of the level expansion circuit. Generally, when the recovery time is long, the noise modulation phenomenon is easily notable and unpleasant to the ear. On the other hand, when the recovery time is short, the noise is masked and hardly detected by the ear. Accordingly, the noise modulation phenomenon can be improved by reducing the above recovery time. However, there is a limit in reducing the recovery time, since it is necessary to prevent the distortion factor in the level compressed and expanded signal from becoming worse due to a ripple component included within the control voltage at the lowermost signal frequency where the signal compression and expansion is performed. Therefore, there was a disadvantage in the conventional noise reducing apparatus in that it was difficult to greatly improve the noise modulation phenomenon.
Thus, as one method of improving the noise reducing apparatus with respect to the above problems, one of the present inventors has proposed an improved noise reducing apparatus in a U.S. patent application Ser. No. 297,106 filed on Aug. 28, 1981 entitled "NOISE REDUCING APPARATUS". In the proposed noise reducing apparatus, the output signal of the variable gain control circuit in the level compression circuit and the input signal of the variable gain control circuit in the level expansion circuit are respectively divided into low and high frequency bands, within the respective control loop circuit in the level compression circuit and the level expansion circuit. The control voltages are produced for every signal in each of the divided bands, and these control voltages are added. The previously proposed noise reducing apparatus thus used a band dividing and adding method.
For example, the control loop in the level compression circuit of the above previously proposed noise reducing apparatus comprises a lowpass filter and a highpass filter supplied with the output signal of the variable gain control circuit, rectifying circuits respectively connected to each filter, an integration time constant circuit for the low frequency band and an integration time constant circuit for the high frequency band respectively connected to each rectifying circuit, and a circuit for adding outputs of the two integration time constant circuits to supply a voltage to the above variable gain control circuit in order to control the gain of the variable gain control circuit. The control loop in the level expansion circuit has a similar circuit construction as the above control loop in the level compression circuit. However, the input signal of the control loop in the level expansion circuit is a signal which is transmitted and applied to the variable gain control circuit in the level expansion circuit, after being recorded and reproduced (or transmitted and received). The signals which are banddivided at the two filters are converted into control voltages corresponding to their respective envelopes, by being passed through the rectifying circuits and the integration time constant circuits.
However, in the above previously proposed noise reducing apparatus, when the level compression and level expansion are simply performed with a flat frequency characteristic, there is a problem in that the noise modulation phenomenon due to noise distributed in the high band is not reduced.
Accordingly, as an apparatus for solving the above problem, an apparatus may be considered in which the level compression is performed after performing pre-emphasis with respect to the input signal within the level compression circuit, and the level expansion is performed after performing de-emphasis with respect to the signal within the expansion circuit. Generally, in the compression and expansion circuits to which the above pre-emphasis and de-emphasis circuits are to be applied, the control voltage generating loop is provided with a weighting circuit having a weighting characteristic closely resembling the emphasis characteristic, at a stage before the rectifying circuit. That is, a level difference corresponding to the emphasis characteristic, is given between the output signal levels of the lowpass filter and the highpass filter.
However, in this apparatus what can be considered a balance cannot be obtained with respect to the relationship between the attack time and the recovery time of the integration time constant circuit for the low frequency band and the attack time and the recovery time of the integration time constant circuit for the high frequency band. Hence, it becomes difficult to set the desired integration time constant. Moreover, there is a disadvantage in that sufficient effect cannot be obtained in reducing the noise modulation phenomenon.
Detailed description will be given on why sufficient effect cannot be obtained in reducing the noise modulation in the above apparatus which can be considered. The above rectifying circuits and the integration time constant circuits are constructed so that an input signal e.sub.i is rectified and integrated at a circuit comprising a resistor R.sub.1, an internal resistance R.sub.dF of a rectifying diode D, a resistor R.sub.2, and a capacitor C. During the positive half wave of the input signal e.sub.i, a current flows to the capacitor C, through the resistor R.sub.1 and the internal resistance R.sub.dF of the diode D in a series manner, to charge the capacitor C. On the other hand, during the negative half wave of the input signal e.sub.i, the electrical charge stored in the capacitor C is discharged through the resistor R.sub.2 connected in parallel with the capacitor C. Hence, during the negative half wave of the input signal e.sub.i, a control voltage E.sub.c is generated between terminals of the resistor R.sub.2.
The forward voltage V.sub.F versus forward current I.sub.F characteristic of the diode D shows a characteristic wherein the voltage V.sub.F and the current I.sub.F is proportional over a predetermined voltage. Accordingly, with respect to a signal current i.sub.dF flowing through the diode D, the characteristic of the internal resistance R.sub.dF is a characteristic wherein the current i.sub.dF and the resistance R.sub.dF is proportional over a predetermined current. Thus, an attack time T.sub.a of the above rectifying and integrating time constant circuits becomes equal to the charging time constant of the capacitor C, and can be described by the following equation (1). EQU T.sub.a =(R.sub.1 +R.sub.dF).multidot.C (1)
Moreover, a recovery time T.sub.r becomes equal to the discharging time constant of the capacitor c, and can be described by the following equation (2). EQU T.sub.r =R.sub.2 .multidot.C (2)
Therefore, the attack time T.sub.a is affected by the internal resistance R.sub.dF of the diode D. As described above, this internal resistance R.sub.dF varies according to the magnitude of the signal current i.sub.dF flowing through the diode D in the forward direction. The attach time T.sub.a is short when the input signal level is high, and long when the input signal level is low, and is substantially inversely proportional with respect to the level of the input signal e.sub.i. On the other hand, as shown in the above equation (2), the recovery time T.sub.r assumes a constant value regardless of the level of the input signal e.sub.i.
When the circuit is constructed to give a level difference between the output signal levels of the lowpass filter and the highpass filter, the following relation indicated by the equation (3) is obtained, according to the non-linear characteristic of the rectifying diodes and the equations (1) and (2). In the equation (3), T.sub.aL and T.sub.rL respectively indicate the attack time and recovery time of a circuit supplied with the output signal of the lowpass filter, and T.sub.aH and T.sub.rH respectively indicate the attack time and recovery time of a circuit supplied with the output signal of the highpass filter. ##EQU1## The above equation (3) indicates that the ratios between the attack time and the recovery time in the circuit supplied with the output signal of the lowpass filter and the circuit supplied with the output of the highpass filter become unequal. Accordingly, the relationship between the attack time with respect to the desired recovery time becomes complicated according to the non-linear characteristic of the rectifying diode. Thus, it becomes difficult to set the integration time constant. The integration time constant is closely related to the noise modulation phenomenon, and as a result, this noise reducing apparatus which can be considered had a disadvantage in that an expected effect could not be obtained in improving the noise modulation phenomenon.